Ball mills are commonly used as one step in the process of reducing the size of ore in commercial mining operations. A ball mill typically consists of a large cylindrical drum mounted on bearings for rotation about a substantially horizontal axis and driven by a powerful motor through conventional reduction gearing. The axial ends of the drum are open, and the ore which is to be comminuted is continuously fed into the mill at one end with the product of reduced size continuously emerging from the other end.
In conventional ball mills, comminution occurs by the balls falling and tumbling onto the ore fragments as the drum is rotated. In an operation of this type, 40-50 percent of the overall charge consists of balls. The term "ball mill" also encompasses a semiautogenously operated mill, in which 2-15 percent of the total charge is balls. In a semiautogenous operation, part of the ore is comminuted by the balls, and part is self comminuted.
Generally, several steps are required to reduce the ore from the larger, randomly sized fragments resulting from the mining operation. Each step requires large, heavy and sometimes complex equipment which represents a high initial cost and requires substantial energy (usually electrical) to operate. Ball mills, some of which are operated semiautogenously, constitute one type of equipment. Other types include rod mills, autogenous mills (in which the ore is self-comminuted by tumbling in a drum or the like), gyratory crushers and roll crushers. Each of these different types of machines is ordinarily used to reduce the ore fragments or particles in a particular size range, and all may be necessary in a particular comminuting process.
It is presently known to employ more than one reducing zone in ball mills. The multizone concept increases the efficiency of the comminuting process because it performs more than one reducing step with a single piece of equipment. However, in order to effect the proper gradation of the ore fragments or particles, and to prevent larger particles from passing into subsequent zones before being properly reduced in size, these ball mills are compartmented through the use of sizing screens. Accordingly, the fragments are tumbled in a particular zone until they have been reduced to a size which permits them to move through the grading screen to the next zone.
As of the present time, the multizone concept has not been possible with semiautogenous mills. Whereas the usage of balls increases the efficiency of a comminuting process by crushing, nipping and rolling the ore fragments to a reduced size, they tend to destroy any grading screen which is used to retain the fragments in a given zone. Accordingly, to my knowledge, there are no multiple zone semiautogenous mills in the prior art.